Tweaks to build-all scripts

clear-flags.ps1

@@ -0,0 +1,7 @@
+# Remove the override of PLATFORMIO_BUILD_FLAGS
+# This is useful if you were running build-all.ps1 and
+# had to kill it with Ctrl-C, thus leaving
+# PLATFORMIO_BUILD_FLAGS set to override the machine type.
+# A clear-flags.sh equivalent is unnecessary, as build-all.sh
+# does not set PLATFORMIO_BUILD_FLAGS globally
+Remove-Item Env:PLATFORMIO_BUILD_FLAGS